1. Field
The present disclosure relates to a cathode active material, a cathode including the cathode active material, a lithium battery including the cathode active material, and methods of preparing the cathode active material, and more particularly, to a cathode active material that provides improved discharge voltage capacity retention and cycle life characteristics.
2. Description of the Related Art
A lithium battery such as a lithium ion battery (“LIB”) has been adopted as a power source for many portable devices due to its high energy density and its simple design. In recent years, an LIB has been adopted as a power source for electric vehicles and electricity storage in addition to portable information technology (“IT”) devices, and studies regarding materials for achieving a high energy density or a long-life of an LIB are being expanded accordingly.
As cathode active materials for a lithium battery, transition metal oxides such as LiNiO2, LiCoO2, LiMn2O4, LiFePO4, LiNixCo1-xO2 (where 0≦x≦1), and LiNi1-x-yCoxMnyO2 (where 0≦x≦0.5 and 0≦y≦0.5) and lithium are used.
However, LiCoO2 is relatively expensive and its specific capacity is about 140 milliampere hours per gram (mAh/g), and thus LiCoO2 has a limited electrical capacity. When LiCoO2 is used at an increased charge voltage up to 4.2 V or more, 50% or more lithium is removed to provide Li1-xCoO2 (where x>0.5). An oxide in the form of Li1-xCoO2 (where x>0.5) is structurally unstable, and its capacity rapidly decreases with subsequent charge and discharge cycles.
Cathode active materials such as LiNixCo1-xO2 (where x<1) or LiNi1-x-yCoxMnyO2 (where 0≦x≦0.5 and 0≦y≦0.5) have decreased structural stability at high voltages.
A lithium metal oxide in the form of Li[LixK1-x]O2 (where x>0 and M′ represents a plurality of transition metals) has theoretically a specific capacity of about 250˜280 mAh/g, and could provide increased electrical capacity. However, a lithium metal oxide including an excessive amount of lithium has decreased structural stability at a high voltage so that a discharge voltage is rapidly decreased and its cycle life characteristics become poor.
Therefore, there remains a need for a new cathode active material that has high capacity, inhibits a discharge voltage drop, has improved structural stability at a high voltage, and has improved cycle life characteristics, and a method of preparing the new cathode active material.